The invention relates to a high density stock pump, and method of operation thereof, especially for use with equipment in pulp mills. In the past, for low consistency pulp (about 5% or less) ordinary stock pumps were used to transport the pulp from stage to stage, and where high consistency pulp (about 8% or greater) was present gravity feeds were normally used to effect transport of the pulp. However, with the recent advent of medium and high consistency components for all phases of pulp treatment, thick stock pumps have become desirable since they allow flexability in planning of a mill, and efficient utilization of equipment. A number of conventional thick stock pumps are on the market for pumping high consistency material, and such pumps in general perform their intended functions very effectively. However, in some installations and with some sources of supply, a large amount of tramp metal is intermixed with a high consistency material. The tramp metal can quickly result in damage to the thick stock pump rotors and blades. This problem can be minimized, all though not entirely eliminated, by utilizing chip cleaning systems; however such chip cleaning systems are expensive to install and operate. Additionally, prior art systems are not successful for solving the tramp metal problem to the extent that thick stock pumps can be placed directly in the digester discharge line, but by utilizing a pump according to the present invention it is possible to place a thick stock pump directly in the digester discharge line.
According to the present invention, the tramp metal problem in conventional pulp mills is handled in a simple and effective manner by making relatively simple modifications to conventional thick stock pumps. According to the present invention, a pump is provided for pumping high consistency material comprising means for defining a closed pumping chamber including an inlet and an outlet; a pair of rotors; means for transforming rotation of the rotors into linear pumping of the high consistency materials so that a head of about 10 to 170 ft. can be achieved during pumping and so that damage will not occur to the pump when tramp metal or the like is contained within the high consistency material pump, said means comprising a plurality of blades associated with each rotor and extending radially therefrom which remain rigid during pumping of high consistency material but flex so as to prevent damage to any pump components when tramp metal or the like is encountered thereby; means for rotating the rotors in opposite angular directions, said means including a shaft associated with each rotor; and means for mounting the rotors so that each blade from one rotor sequentially cooperates with but does not substantially engage a circumferential surface of the other rotor intermediate to blades of the other rotor, during continued rotation of the rotors, and so that each blade sequentially cooperates with but does not substantially engage an interior portion of said closed pumping chamber during continued rotation of said rotor.
Each of the pump blades include a slanted tip having a first end portion thereof located a greater radial distance from the shaft of the rotor with which it is associated than the second end portion thereof, and during rotation of the rotors the first tip end portion of each blade is the leading tip portion of that blade. The necessary flexibility while still achieving high consistency material pumping can be obtained constructing the blades in a number of different manners. In one embodiment according to the invention, each blade consists essentially of an elongated substantially integral member of flexible material. In another embodiment, each blade comprises an elongated substantially integral member of flexible material with a backing member of rigid material extending a smaller radial distance from the rotor circumferential surface then the flexible member. According to another embodiment, each blade comprises a tip portion of relatively rigid material and a base portion of relatively rigid material with an intermediate portion that is flexible. Additionally, instead of employing an inherently flexible material such as rubber or TEFLON for the blades, a set of thin blades or laminates of a springy flexible metal can be provided, either forming the entire blade or inner connecting rigid portions of the blade. Under circumstances where the tramp metal problem is especially severe, relatively large clearances can be provided between each blade from one rotor in the circumferential surface of the other rotor with which it cooperates, and the interior portion of the closed pumping chamber, so that tramp metal may pass between each blade and its cooperating other rotor circumferential surface, and the blade and the interior chamber portion.
The pump according to the present invention is especially useful in lines interconnecting vessels adapted to contain high density cellulosic pulp (at a consistency of greater than about 8%). According to a general combination of the invention, a first vessel containing high density pulp and having an outlet is connected by a connecting line to a second vessel for receiving high density cellulosic pulp, with the pump disposed in the connecting line for pumping the high density material from the first vessel outlet to and thru the second vessel. The first vessel preferably comprises an oxygen reactor (a pressure release mechanism being provided in the interconnecting line), and the second vessel is a diffusion washer. Other advantageous combinations include both the first and second vessels being diffusers, or the first vessel being a high density storage tank and the second vessel being a suitable further component.
By utilizing the pump according to the present invention, a method of pumping high density material having a consistency of greater than about 10% with a head of about 10-170 ft. maybe practiced without damage to the pumping mechanism.
It is the primary object of the present invention to provide a thick stock pump that will not be damaged by tramp metal in the high consistency material being pumped. This and other objects of the invention will become clearer from an inspection of the detailed description of the invention, and from the appended claims.